Date of Award

Summer 2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)

Department

Committee for the Interdisciplinary Program in Materials Science and Engineering

First Advisor

Trevor Tyson

Second Advisor

N. M. Ravindra

Third Advisor

Ken Keunhyuk Ahn

Fourth Advisor

Tao Zhou

Fifth Advisor

Mark C. Croft

Abstract

Complex oxides such as the manganites exhibit an intimate coupling of the electron spin, charge and lattice degrees of freedom. The characteristic feature of these materials is the existence of a ground state topology with many closely lying minima. The system can be switched from one minimum to another by the application of external parameters such as strain, temperature, magnetic fields, or electrical fields. These materials thus exhibit large responses to these external parameters and can be used as novel sensors in data storage and other applications. Understanding the couplings will have an impact on the fundamental science of highly correlated materials as well as in applications to industry.

This work focuses on applying pressure to probe the properties of Colossal Magnetoresistance Manganites, self-doped LaxMnO3 (X=0.85, 0.75) and chemically doped La0.67Ca0.33MnO3. For all of them, obvious pressure effects on electronic transport and structure have been investigated; especially, critical pressures were found between ~3.4 GPa to ~4.0GPa. Below P*, pressures increase the metal-insulator transition temperature and electrical conductivity while both of them decrease with increasing pressures above P~. In particular, the bandwidth increase drives the increase of TMI for pressures below The reduction of TMI at higher pressures is found to result from the Jahn-Teller distortions of the MnO6 octahedra and the localization of 3d electrons. The general trend is expected to be a characteristic feature of Colossal Magnetoresistance manganites.

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